Group A Streptococcal disease as shown by the rate of infections by age group is a childhood disease (1-3). Much like the other diseases in this category such as meningococcal (4) and Haemophilus meningitis (5), diphtheria (6) and others (7,8), the majority of cases occur in young children and the rate of infection decreases with age. Thus, by the age of eighteen years, the incidence of group A Streptococcal infections is relatively low (1-3). This would suggest that some type of natural immunity to this group of organisms may occur over time much like that found with other childhood infections.
In experiments extending over several decades, Lancefield and colleagues (9-11) established that the vast majority of hemolytic streptococci infecting humans were group A. This distinction was based on serological reactions to group A Streptococcal carbohydrate. Later studies reported that the immunodominant determinant was N-acetylglucosamine (12,13). Using mouse protection tests and precipitin assays, these group A Streptococci were further sub-divided into serotypes based on the presence of antigenically different M proteins present on the surface of the organism. It has been clearly shown that antibodies directed against a specific M serotype are protective in a mouse model of infection (14). In humans, recovery from group A Streptococcal infection is often associated with long lasting immunity which is type specific to the infecting organism (11). But in both cases, the protection is M serotype-specific and does not extend to protection against other serotypes. In addition, it has been demonstrated in numerous studies that human sera rarely contain multiple M protein serotype specific antibodies (11,15). These classical experiments, both in humans and experimental animals, established an important role of the M protein in the virulence of group A Streptococci and have formed the basis for numerous unsuccessful attempts to develop streptococcal vaccines that would elicit protective antibodies either toward the amino-terminal portion of the M protein in which the serotypic specificity resides or more recently to the common C-repeat regions of the molecule (16).
However, in view of the age related nature of group A infections which suggests a rise in natural immunity to this group of bacteria, the question remains whether this represents a slow rise in antibodies directed at more common regions on the M protein or whether other surface antigens which have received less attention might play a role in this naturally acquired nonserotype specific protection. For example, the hyaluronic acid capsule plays an important role in the virulence of group C infections in guinea pigs (17) and anti-hyaluronate antibodies have been detected in animals (18, 34) and humans (19). Hyaluronic acid from group A Streptococci was reported as being immunogenic in rabbits after immunization with formalized, encapsulated group A Streptococci or bound to liposomes (18). Use of liposomes in vaccines has also been reported (31). Injection of the mucopeptide fractions of the streptococcal cell wall induces a short lived protection in experimental animals (20) but its role in humans remains unknown.
The group specific carbohydrate consists of a poly-rhamnose backbone to which, in the case of Group A, an N-acetylglucosamine is present at the non-reducing terminal position (FIG. 1a). Group A variant streptococci have been described and characterized (12,13). In these streptococci, the poly-rhamnose backbone is present but remains undecorated by N-acetylglucosamine (FIG. 1b). In early experiments, rabbits were injected with whole group A Streptococci lacking M protein and this was shown to elicit precipitating antibodies to the group A carbohydrate. However, these antibodies were not passively protective against an M protein positive group A Streptococcal challenge in passive mouse protection studies (14). Furthermore, several earlier attempts to demonstrate similar precipitating antibodies in humans were unsuccessful, suggesting that precipitating carbohydrate antibodies did not play a significant role in protection against streptococcal infections.
However, because most of the early methods to detect a rise in antibodies depended on the ability of these antibodies to become precipitable with the addition of antigen, many antibodies which none-the-less were reactive with a specific antigen but did not precipitate in such assays, were left undetected. Antibodies reactive to the hyaluronic acid capsule of group A Streptococci provide one good example. In studies focused on eliminating this problem, a series of reports beginning in 1965 (21,22) employed both direct and indirect agglutination techniques to detect antibodies. Direct agglutination detected precipitating antibodies while the indirect agglutination would measure both precipitating and non-precipitating antibodies. Of interest was the demonstration by Karakawa et al (22) that the direct agglutinating antibodies, i.e. the precipitating antibodies, in these human sera were directed primarily against group A variant carbohydrate, the poly-rhamnose backbone, while the indirect agglutination techniques directed at the non-precipitating antibodies detected a high titer of antibodies to the N-acetylglucosamine determinant.
Subsequent studies by Zimmerman et al (23), employing human sera from a variety of streptococcal infections, indicated that the incidence of these non-precipitating antibodies varied from a low of 30% in a population, which had been carefully followed and treated for streptococcal infections, to a high of 84% in a population recently infected with group A Streptococci. They also noted that antibody titers to the group A carbohydrate peaked at age 17 and that there was no difference in antibody titers to this carbohydrate in rheumatics with and without heart disease. These results differed from those reported by Dudding and Ayoub in which anti-group A carbohydrate antibodies were persistently elevated in rheumatic heart disease patients compared to those without valvular damage (24).
The question of whether or not these antibodies play a role in protection has been difficult to assess. The classic opsonophagocytosis assay of Lancefield used selected whole human blood (15,25,26) to which hyperimmune rabbit sera with known M protein serotype specific antibodies were added. The selection of the whole human blood was based on two facts; (1) it contained no M protein reactive antibodies and/or (2) would not promote phagocytosis of streptococci in the absence of the serotype specific rabbit antiserum. The question whether the normal human sera per se could enhance phagocytosis was never really addressed.